1. Field of the Invention:
The present invention relates to circuits for monitoring a three-phase A.C. power source in order to detect a missing phase voltage, or certain other malfunction conditions. It is of special utility where the three-phase source provides power for electronic circuits operating in a digital mode of control, and in these applications can turn the electronic circuits off (to prevent damage thereto) with response times of the order to milliseconds, and can provide automatic recovery of circuit operation from non-permanent malfunctions also with response times of the order of milliseconds.
2. Description of the Prior Art
The requirements placed upon the power supply and power supply monitoring equipment for military aircraft have become increasingly stringent under the trend toward more and larger electronic and electromechanical systems aboard the aircraft. The needs of the power supply have been met by constant speed drive alternator equipment which has provided a source of high power 400 cycle, three-phase, A.C. power aboard such aircraft. However, there has been a continuing effort to satisfy certain requirements for monitoring such power sources which are a consequence of these and other technological trends. For example, along with the development of the aircraft high power A.C. power sources, there has been the development of high power A.C. to D.C. semiconductor converters which use semiconductor components having relatively short thermal time constants, which components can therefore become overstressed in periods as short as milliseconds. There has also been continuing interest in the improvement of monitoring equipment of the type which turns off equipment subject to damage, to give such equipment the further capability of automatic recovery in order that the various complex systems required for military aircraft be available in times of combat should the systems be interrupter by transistory or non-permanent malfunction indications.
In addition to the above-mentioned interests and needs, there is a continuing need to provide power source monitoring circuits which are of lighter weight, and are manufacturable with improved ease of reproducibility.
The use in monitoring circuits for the detection of missing phase conditions of sense channels employing rectification is known. One example is U.S. Pat. No. 3,001,100 entitled "Undervoltage Sensing Circuit" and which has the same assignee as hereunder; and another is U.S. Pat. No. 3,758,826 entitled "Photoelectronic Safety Device" to G. Zizola. Both of these circuits employ the principle of rectification and subsequent filtering of the rectified signal. A disadvantage of this mode of operation is that the time response to the device then becomes limited by the filtering, so that the time response is always long relative to the period of the phase signal. Also, while these devices do have a missing phase detection capability, this capability depends upon a calibration adjustment, thereby causing additional work in the manufacturing and maintenance of the device. Further, the filter structure involves use of a relatively heavy and bulky capacitor.
Another prior art patent of interest is U.S. Pat. No. 3,136,920 entitled "Power Failure Indicator" to R. C. Jensen. This patent uses the threshold of operation of neon glow tubes to sense reduced voltage conditions. An inherent result of the Jensen's structure is the provision of optoisolation of the input stage and output stage of a monitoring circuit for detecting a power source malfunction.